The chief medical examiner's spokeswoman, Ellen Borakove, said the area first will be tested as part of a standard health and safety evaluation for possible toxicity. She said sifting for human remains is to begin Tuesday morning.

Retired fire department deputy chief Jim Riches, who lost his son in the terror attack, visited the site on Saturday. He said the latest news left him feeling "upset."

"The finding of this landing gear," he said, "just goes to show that we need federal people in here to do a comprehensive, full search of lower Manhattan to make sure that we don't get any more surprises," as happened in 2007 when body parts were discovered in nearby sewers and manhole covers.

Of the nearly 3,000 victims, Riches noted, about 1,000 families have never recovered any remains.

The New York Police Department has declared the alley, between the mosque site and a luxury loft rental building, a crime scene where nothing may be disturbed until the medical examiner's office completes its work. It's unclear how long that may take, Borakove said.

The piece of wreckage was discovered Wednesday by surveyors inspecting the planned Islamic community center, known as Park51, on behalf of the building's owner, police said.

The landing gear was wedged between the back walls of the apartment building and the mosque, which once prompted virulent national debate about Islam and free speech.

Chicago-based Boeing spokesman John Dern could not confirm whether the ID matched the American Airlines plane or the United Airlines plane hijacked by Islamic extremists on Sept. 11, 2001. He said Boeing has been asked to take part in the examination of images of the airplane part by the National Transportation Safety Board, which is providing assistance to New York authorities overseeing the probe.

The twisted metal part — jammed in an 18-inch-wide, trash-laden passageway between the buildings — has cables and levers on it and is about 5 feet high, 17 inches wide and 4 feet long, police Commissioner Raymond Kelly said Friday.

"It's a manifestation of a horrific terrorist act a block and a half away from where we stand," he said after visiting the alley.

The commissioner noted that a piece of rope intertwined with the part looks like a broken pulley that may have come down from the roof of the Islamic community center.

When plans for the center became public in 2010, opponents said they didn't want a mosque so close to where Islamic extremists attacked, but supporters said the center would promote harmony between Muslims and followers of other faiths.

The building includes a Muslim prayer space that has been open for three years. After protests died down, the center hosted its first exhibit last year. The space remains under renovation.